Water-resistant synthetic papers mainly comprising resins have found their use chiefly in offset printing or seal printing using oily inks or UV-curing inks and sublimation or melt type thermal transfer. With broadening of application, there has been an increasing demand for synthetic papers suited to printing with aqueous inks and application of environmentally friendly aqueous pastes, that is, synthetic papers exhibiting satisfactory absorbing properties for aqueous ink, aqueous pastes or water as a medium thereof.
Aided by recent technical advances in multimedia, ink jet printers have become wide spread in both business and domestic use. Ink jet printers have many merits, such as adaptability for multicolor printing, capability of forming large images, and low printing cost. In particular, ink jet printers using aqueous inks have supplanted those using oily inks in view of environmental and safety considerations.
Ink jet printers have now found wide use as a means for obtaining hard copies not only from word processors but from image processors as well. It has therefore been required for printed images to have more precision. Image precision depends on drying properties of ink applied to recording media. For example, when a plurality of recording sheets are printed and superposed on top of another in succession, the printed ink may cause offset to stain the image if the recording sheets have insufficient ink absorptivity.
To improve image precision, it is a commonly followed practice to coat a recording medium, such as synthetic paper, plastic film or pulp paper, with an ink receptive material containing a hydrophilic resin or inorganic fine powder (see, for example, JP-A-3-82589 and JP-A-9-216456) Ink jet recording media having an ink receptive layer mainly comprising a synthetic resin provided by thermal lamination or extrusion lamination have also been proposed (see JP-A-8-12871, JP-A-9-1920 and JP-A-9-314983). However, pulp paper coated with an ink receptive layer is liable to develop surface unevenness on the printed surface where a large amount of ink is ejected. Because plastic films coated with an ink receptive layer tend to lack enough absorptivity when a large amount of ink is ejected, the coating must have an increased thickness, requiring a number of coating operations.
Accordingly, an object of the present invention is to solve the above problems associated with the related art by providing a porous film having satisfactory absorptivity for water as a solvent of aqueous ink or aqueous paste.
Another object of the invention is to provide a recording medium, especially an ink jet recording medium, which uniformly absorbs aqueous ink without causing ink density unevenness even when solid areas are printed with a large amount of ejected ink.